The invention relates to a novel CO2 sorbent material. More particularly, the invention relates to a novel CO2 sorbent, to a method of making the novel sorbent material, and further to a process for using the same.
CO2 absorbers allow for the capture of CO2 from CO2-containing streams, and then concentrating the effluent to a nearly 99.9% CO2 stream. CO2 capture is important given that exposure to carbon dioxide partial pressures exceeding about 7.6 mm Hg, at partial pressure of about 1%, for extended periods of time are known to cause health problems in humans and other mammals. As a result, in enclosed habitable environments such as those existing in a submarine, space craft or space suit, for example, carbon dioxide partial pressures are typically maintained below about 1% via the use of solid regenerable and nonregenerable carbon dioxide sorbents such as soda lime, zeolite and carbon molecular sieves, solid oxides, alkali metal carbonates, alkali metal hydroxides, amines and combinations thereof.
These solid regenerable carbon dioxide sorbents have also been considered for capturing CO2 from the flue gas of coal-fired power plants. At present, the quantity of CO2 emitted from coal-fired power plants accounts for more than 36% of total global CO2 emission and represents the largest stationary CO2 emission source. Depending on the combustion conditions and the type of coal used, the volume fractions of CO2 in flue gas ranges from 10 to 15%. The development of a cost-effective approach to remove CO2 from the flue gas of coal-fired power plants has been adopted as one of the short term strategies in stabilizing the atmospheric CO2 concentration.
In addition to these sorbents, amines such as monoethanolamine and diethanoamine are often used in a liquid phase to reduce carbon dioxide partial pressures via absorption. These amines are utilized in the aqueous phase, typically at a concentration of 25 wt. % to 30 wt. % amine. In known capture systems, the amine solution enters the top of an absorption tower while the CO2 containing gaseous stream is introduced into the bottom of the tower. While intimately contacting the gaseous stream in a counter-current fashion, the amine solution chemically reacts with the carbon dioxide to absorb and remove the carbon dioxide from the gaseous stream. Desorption of the absorbed carbon dioxide then proceeds via a thermal regeneration process at temperatures in excess of about 150° F. (about 66° C.). During desorption, carbon dioxide and water evolve from the amine solution and are separated by condensing the water vapor in a heat exchanger. Once regenerated, the amine solution is recycled back to the absorption tower for additional carbon dioxide absorption.
Unfortunately however, while amine solutions are capable of initially removing carbon dioxide to partial pressures below about 1%, they have a limited life due to degradation through oxidation of the amine. Oxygen present in the gas stream oxidizes the amine, particularly at the elevated temperatures during desorption when the amine solution is heated. This oxidation tends to reduce the amount of amine primary and secondary functional groups available for carbon dioxide absorption. Consequently, the amine solution's useful life is limited to only about six months of continuous use.
If desorption is accomplished at ambient temperatures, the useful life of the amine solution may be extended, however performance will be limited by low desorption rates. Due to both energy requirements and oxidation related degradation, amine sorbents utilized in closed environment systems are often regenerated at approximately ambient temperatures for a fixed desorption time. At ambient temperatures, the desorption of carbon dioxide is often limiting, taking many hours to complete a full desorption which most systems and uses cannot afford. Under conditions, at which full desorption is not achieved, a portion of the absorbed carbon dioxide remains in the sorbent after the desorption process is halted, thereby reducing the capacity of the sorbent to absorb additional carbon dioxide through a subsequent cycle.
U.S. Pat. No. 5,376,614 discloses a solid aminepolyol sorbent comprising about 1 weight percent to about 25 weight percent amine, about 1 weight percent to about 25 weight percent polyol, and the remainder being a porous support which provides the amine with structural integrity and a high surface area for gas/solid contact. While the solid amine sorbent disclosed in this reference provides a number of advantages over the previously described solid and liquid sorbents, increased concentration of amine in the sorbent and improvements in cyclic CO2 removal capacity are still needed to optimize system performance. In addition, formation of the sorbent in the '614 process is a fairly complex, requiring either pre-forming an amine/polyol solution and then impregnating the support with this solution, or impregnating the support with an amine solution followed by a second impregnation of the support with a polyol solution.